Resolves issue #20. Additionally, load_session now has an optional
second argument to set the default session name. And error output from
both load commands is now printed to STDERR and give a return code of 1
on error.
This means that "init" session layouts can now be used to start and/or
re-attach to your main session.
Personally I have a "main" session layout with a couple of default
windows. I can now either start that session, or re-attach to it from
outside of Tmux by just running `tmuxifier s main`, instead of manually
creating a session named "main" and then loading my windows into it.
* `session` and `window` commands are now know as `load-session` and
`load-window` respectively, with aliases for their former names.
* Add short aliases for various commands.
* `help` and `completions` commands have been expanded with support for
aliases.
* `help` command has been expanded upon with brief docs for a few more
commands.
